The present application is based on and claims priority from the following Japanese Patent Applications: 2001-48714, filed Feb. 23, 2001; 2001-60649, filed Mar. 5, 2001; and 2001-70870, filed Mar. 13, 2001; the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an indicating instrument for a vehicle, such as a passenger car, a truck, a bus, or a motor cycle which has a step motor as the driving power source thereof.
2. Description of the Related Art
Usually, this type of the indicating instrument has a speed reduction gear train connected to the step motor and disposed at the back of a dial plate thereof. When the step motor drives the gear train by an input shaft thereof, an output shaft of the gear train rotates a pointer shaft to move a pointer over the front surface of the dial plate.
The indicating instrument includes a stopper unit that has a stopper and an arm. Such a stopper is a projection formed at a portion of an output gear of the gear train that corresponds to a zero-position or a reset position of the dial plate. The arm is supported by a stationary member at the back of the dial plate so that the edge thereof can engage the stopper when it returns to the zero-position. Thus, the stopper unit stops the pointer at the zero-position by the stopper.
If the pointer is returned to the zero-position, cosine wave voltage is applied to the step motor. Subsequently, the rotor of the step motor starts rotating or reversing toward the zero-position, and a voltage is induced in a field coil of the step motor. As the rotor rotates faster, the induced voltage becomes higher. When the induced voltage becomes lower than a threshold voltage, it is presumed that the pointer has returned to the zero-position. Consequently, the cosine wave voltage is stopped.
However, the rotation speed of the rotor is so low just after the cosine wave voltage is applied that the amplitude of the induced voltage is not sufficient to be compared. If this voltage is compared with a certain threshold voltage, the zero-position of the pointer may not be correctly judged.
Therefore, an object of the invention is to provide an indicating instrument in which a pointer can always return to a zero-position so that the pointer can indicate an accurate analog value, such as a vehicle speed, that is sensed by a sensor.
According to a feature of the invention, an indicating instrument for a vehicle includes a dial plate having a pointer disposed to move over the display portion between a zero position and a maximum position corresponding to the maximum value, a step motor including a stator for providing a rotating magnetic field corresponding to the analog value, and a magnet rotor connected to the pointer for moving the pointer in response to the rotating magnetic field, a stopper unit for stopping the pointer when the pointer reaches the zero position, zero-return means for supplying the stator with a zero-return alternating signal to move the pointer to the zero-position, storing means for pre-storing a zero-level electric angle in which the zero-return alternating signal becomes a zero level at least twice after the zero-return signal is supplied to the stator, first examining means for examining whether the phase angle of the zero-return alternating signal reaches the zero-level electric angle or not after the zero-return alternating signal is supplied to the stator, interrupting means for interrupting the zero-return alternating signal to provide induced voltage induced in the stator when it is presumed that the zero-return alternating signal reaches the zero-level electric angle, and second examining means for examining whether the induced voltage is lower than a threshold voltage that indicates that the pointer is stopped.
The indicating instrument may include a speed reduction gear train that is connected between the magnet rotor and the pointer so that the pointer can rotate at a reduced rotation speed proportional to a rotation speed of the magnet rotor.
The indicating instrument may further includes means for providing a pair of alternating signals, and the stator may be comprised of a pair of field coils for forming the rotating magnetic field when the pair of alternating signals are supplied thereto.
According to another feature of the invention, an indicating instrument for a vehicle includes a dial plate, a pointer, a step motor including a stator and a magnet rotor, a stopper unit for the pointer, drive means for supplying a stator of the step motor with a drive alternating signal, zero-return means for supplying the stator with a zero-return alternating signal, storing means for storing a phase angle of the zero-return signal as a zero-return angle correction value if an induced voltage, which is induced in the stator when the zero-return alternating signal is interrupted after the zero-return alternating signal becomes a zero level at least twice, becomes lower than a threshold value that indicates that the pointer is stopped, adjusting means for adjusting the zero-return alternating signal to zero phase by the zero-return angle correction value when the zero-return alternating signal is supplied to the stator, synchronizing means for synchronizing the zero-return alternating signal and rotation of the magnet rotor after the phase of the zero-return alternating signal is adjusted, first examining means for examining whether the phase angle of the zero-return alternating signal reaches a zero-level electric angle that corresponds to a zero level of the zero-return alternating signal at least twice or not after the alternating signal is supplied to the stator, and second examining means for examining whether the induced voltage is lower than a threshold voltage that indicates that the pointer reduction gear train is stopped. The drive means supplies the drive alternating signal at the phase thereof immediately when the second examining means judges that the induced voltage is lower than the threshold voltage.
The indicating instrument may further include a speed reduction gear train connected between the pointer and the pointer for moving the pointer at a reduced rotation speed proportional to a rotation speed of the magnet rotor or an accelerating means for decreasing phase angle of the zero-return alternating signal to accelerate the magnet rotor.
The indicating instrument may further include means for examining whether phase of the zero-return signal increases by a predetermined angle or not after the adjusting means adjust the zero-return alternating to zero phase, and means for decreasing phase of the zero-return signal to a phase that corresponds to the zero-return angle correction value.
Another object of the invention is to provide a system for setting and storing a zero-return correction angle of an indicating instrument. The zero-return correction angle, which varies due to variation in size of the parts and in assembling steps thereof, is provided for each indication instrument.
According to another feature of the invention, a system for setting a zero-return correction angle of a pointer of an indicating instrument is comprised of first means for adjusting a drive alternating signal to zero phase when the alternating signal is supplied to a step motor of the indicating instrument, second means for examining whether phase angle of the drive alternating signal reaches a zero-level electric angle that corresponds to a zero level of the drive alternating signal at least twice or not after the drive alternating signal is supplied to the step motor, third means for provide an induced voltage in the stator of the step motor induced by the magnet rotor of the step motor when the second means judges that the phase angle of the alternating signal reaches the zero-level electric angle.